AbstractDuring
this lesson students will observe a simulation demonstrating
the difference between connection and inversion. Students
then explain when and where convection and inversion layers
occur and how each impacts air quality, and by connection,
human health.

ObjectivesStudents
will be able to:
1. Observe a classroom demonstration that illustrates
convection currents and temperature inversions.
2. Conduct a classroom investigation that demonstrates
how warm air rises.
3. Make a jar of smog to demonstrate how contained smoke
will not disperse.

National Science
Education Standard: Content Standard
D - STRUCTURE OF THE EARTH SYSTEM

The atmosphere is a mixture of nitrogen, oxygen, and trace gases that
include water vapor. The atmosphere has different properties at different
elevations.

Arizona Science Education Standards
Strand 6 - Earth Science
Concept 1 – Structure of the Earth
PO 1. Describe the properties and the composition of the
layers of the atmosphere.

Teacher
BackgroundOnce
you have lived in Tucson/Pima County for a while, you become
familiar with the haze that sometimes forms over our city.
A drive to larger cities reveals persistent smog with the
whole city sitting in a brown haze. What causes this brown
haze or smog that can cause burning, itching eyes and shortness
of breath? And why does it appear in greater amounts in
regions surrounded by mountains?

Sources of smog may include motorized vehicles, industries,
airplanes, trains, wood stoves, wildfires and blowing dust.
In Tucson/Pima County, approximately 70 % of the air pollution
is caused by motor vehicle use. To begin to understand
where smog is more likely to linger, we must understand
convection currents and temperature inversions. Warm air
is lighter (less dense) than cold air. As warm air rises,
cold air moves to take its place. This cyclic nature of
moving air is called a convection current. Convection causes
currents of air to move around outdoors (and inside buildings
as well.) Birds float upward on rising currents of warm
air and gliders stay up in the air in the same way.

Convection currents help disperse air, including any pollutants
in the air. This natural force moves polluted air rising
from urban centers and dilutes it in less-polluted air
above. Due to convection, air pollution does not remain
isolated or localized. But a temperature inversion can
obstruct normal convection currents.

A temperature
inversion occurs when a mass of warm air moves over stagnant,
cooler, surface air. This warm air
mass forms a lid over the area, trapping all the polluted
surface air left from the city’s transportation systems,
industries, and homes. If a temperature inversion traps
pollutants, then a visible layer of smog will result. Because
our sparse vegetation allows the ground to cool off nightly
and our city is surrounded by mountains, smog is very likely
to linger until the morning sun warms the air enough to
begin the mixing (convection) cycle.

The following experiment demonstrates convection currents
and a temperature inversion.

Ask students the following question: How does weather
affect air pollution?
Lead them toward the next question if they don’t
bring this up themselves:
How do convection currents and temperature inversions
influence air pollution?
By the end of the lesson students will hopefully
be able to explain the following: Polluted air will
be moved and diluted by convection currents, but
will remain stagnant during a temperature inversion.

1. Divide your students into small groups and see
if they can suggest methods to show how convection
currents and temperature inversions influence air
pollution. Perhaps their ideas will illustrate this
point just as clearly as the following experiments,
and the lesson will stick with them longer if they
do the brainstorming. In case there are few reasonable
suggestions, here’s an activity to help you
out!

2.
Take the top off the shoe box and lay the box on its
side. Cut two holes in the topside of the box (one at
each
end), just large enough for two paper towel tubes. Push
the tubes into the holes and seal the openings with tape
in order to ensure an airtight seal. You have just made
two paper towel tube chimneys.

3. Set a candle in a clay base under one of the paper
towel tube chimneys, pressing the clay firmly into place
to hold it tight. The candle should be at least 2 inches
lower than the chimney. (Make sure the wick is exposed
and upright.)

4. Cover the open side of the box with clear plastic wrap.
Tape the plastic wrap to the front of the box, forming
an airtight seal.

5. Very CAREFULLY, using a long match, (or a match taped
to a pencil) light the candle by putting it down the chimney.
Once the candle is lit, allow the box to warm up for approximately
five minutes.

6. Take a tightly wadded up paper towel and light it with
a match. Let it burn for a few seconds, and then blow it
out. It should be smoking profusely. Note that the smoke
rises (warm air). Now hold the smoking paper down over
the second chimney (without the candle). Record your observations:
The cold (heavier) air above the smoking paper will push
the smoke down through the chimney. The smoke will then
warm, rise toward the candle, and exit the convection box
via the opposite chimney. This demonstrates the cyclic
nature of convection and how warm air rises and cold air
sinks.

7. Now, to simulate a temperature inversion, blow out
the candle, place the ice cubes down both chimneys and
let the box cool down for five minutes.

8. While the box is cooling down, put the heat lamp directly
over one chimney, not blocking it, but making sure the
heat is funneled down into the box.

9.
Drop a smoking wad of paper down the other chimney, and
then place a
piece of paper over this chimney. A temperature
inversion prevents normal convection. The warmer air mass
moves over the cooler ground air and traps it. Compare
the heated chimney with the unheated one. After 30 seconds
of viewing the trapped smoke, lift the unheated chimney’s
cover and watch the smoke escape. Record your observations.

Conclusion
Based
on your observations, does this demonstration support
or reject your hypothesis? Why or why not?

Follow-up1. What, in nature, warms the air like the candle did in
the experiment?

2.
How does Tucson’s geography and weather make
it a prime candidate for temperature inversions?

3. What human activity is most responsible for air pollution
in our community?

4. Recall that pollution lingers during a temperature
inversion, when cool polluted air is trapped under a lid
of warm air. At what time of day or night are these conditions
most likely to occur?

5. What human activity occurs at this time of day or night
that contributes to air pollution?

6. What would cause cold, polluted air to rise and be
diluted? At what time of day or night would you expect
this to happen?

7. If the sun rose later in the day, what effect would
this have on lingering air pollution?

8. At what time of the year would you expect this to happen?

Embedded
Assessment
Students
can be assessed on their initial responses to the opening questions,
informally assessed on their discussion during the demonstration
and on their final responses.

PULSE
is a project of the Community Outreach and Education
Program of the Southwest Environmental Health Sciences
Center and is funded by: